Switch contacts tend to lose proper conductivity over time.  Too little
current flow, not enough juice to the window motor, window stops going up or
down, or seems to labor more than it used to.  Different contact for each
direction, so yes, the window can and may still work in one direction and
not the other.  It helps sometimes to use the switches so they don't oxidize
as much from dissuse over time, but eventually any switch will wear out.
You can pull the door panels off and depending on the switch, clean the
contacts, or replace the switch.  I would pull the door panel, unplug the
switch, attatch a meter, and check for continuity through the switch before
assuming the switch is bad for sure, and if you can find a similar
make/model, might be worth a trip to the local salvage yard if the dealer
wants too much for a replacement.  For that matter, you can start at the
yard, pull a door panel and see how the switch works, before you disect your
own car, if you are the unsure type.

If however, there are other issues with the windows such as clunks, scrapes,
or other odd noises eminating from the internal window mechanisms, switches
may not be the culprit.  So, to start, I would try simple stuff, which for
myself would be the switches.

~Brian

You can also try cleaning the switch contacts by spraying them with "tuner
cleaner" available on-line or from your local Radio Shack.  You probably
should remove the switches from the panel first to avoid getting the cleaner
on the panel.